Hitherto, in this type of the devices, fetus electrocardiogram signals by the scalp lead during the intrapartum period have been mainly used. This device is configured so that, a unipolar lead screw-type electrocardiogram electrode is passed through a birth canal via the opening of a vagina, to thereby be directly attached to a portion of a fetus, exposed to the outside of a uterus, for example, to the fetus head or buttock. Such a technique is disclosed in, for example, the Patent Document (1) by Rosen et al., or the Non-Patent Document (1) by Rosen, which are each described below.
These Patent Document (1) and Non-Patent Document (1) each set forth a measuring method for fetus electrocardiogram waveforms obtained from scalp electrodes. This method, however, is disadvantageous in that it limits the measurement time period to the intrapartum period, and that it increases the risk of causing infection to the fetus because this method is obviously an invasive one.
Likewise, regarding an intrauterine embedding type fetus monitoring device by Horio et al. set forth in the Patent Document (2) as described below, its method is also an invasive one in which a microcapsule is directly attached to a fetus through a uteroscope. Because this method requires an operation through a mother body for attaching the microcapsule to the fetus, it has, virtually, only limited application, and involves similar difficulties to those in the example of the above-described Patent Document (1).
Main methods that have hitherto been proposed for measuring a fetus electrocardiogram through the abdominal wall of a mother body, include a method based on a bipolar lead technique using a small number of electrode couples by Ogawa et al. set forth in the Non-Patent Document 2 as described below, and a method by Greenberg et al. set forth in the Patent Document (3) as described below, wherein a plurality of bipolar leads are combined so as to be mutually perpendicular to the direction of the body axis of a fetus.
These methods have been actively studied until recent years partly because there is the need to sandwich the central axis of a fetus between electrodes in order to reliably measure the fetus electrocardiogram, partly because it is possible to remove irregular noises that are mixed into the electrode couple commonly to their counterparts by taking the potential difference between the electrodes forming the counterparts, and partly because these methods are resistant to noise. However, the bipolar method is limited in data analysis technique. That is, this method requires two electrodes to obtain a single piece of measurement data, and the measurement of fetus electrocardiogram needs to cover the abdominal wall of a mother body with a large number of electrodes, making allowance for movements of a fetus in fetus activities. Moreover, when the fetus is small and hence the direction of the central axis of the fetus is hard to estimate, the measurement is difficult to perform. In particular, when fat components (vernix caseosa) of the fetus increases around the fetus as in the period at 26 to 36 weeks of gestation, and consequently the electric conductivity in the fetus electrocardiogram decreases, measured potentials come down to small values to thereby frequently make measurement impossible. Therefore, measurements by this bipolar method have often been performed in late pregnancy.
Peters M, Crowe J, Hayes-Gill B et al. in England, as shown in the Non-Patent document 3 described below, performed continuous measurements for long time period with five electrodes attached to the abdominal wall of a mother body, and analyzed fetus electrocardiogram signals that happened to be found therein, whereby they developed a device for measuring a fetus electrocardiogram, and attempted to productize it as a portable mother/fetus monitor. This method is an attempt to solve the problem that the measurement rate of the fetus electrocardiogram by the bipolar method is low. However, this method does not meet the condition that medical devices should satisfy. In other words, this method does not meet the performance requirement that the same result should be available whenever, wherever, and by whomever a measurement is performed. Therefore, it is not necessarily the case that this method can be used whenever an examination is needed, and sometimes, there occurs a need to wait one day or more before the result is available. This method, therefore, significantly restricts its availability, and does not provide a monitor usable whenever it is needed.
Furthermore, according to a method disclosed in the Non-Patent Document (4) described below, it is known that, in the bipolar electrode method, during a uterine contraction wherein a uterus contracts and the electromyogram increases, in the case where noise components abruptly change or the base line of signals irregularly increases/decreases during movements of a mother body, the S/N ratio decreases to thereby make the measurement impossible.
With this being the situation, as an alternative method to the bipolar electrode method, a unipolar method has now become mainstream wherein a plurality of measurement electrodes are attached to the abdominal wall of a mother body, and a reference electrode is placed at a position other than the measurement electrode positions on the mother body. The use of the unipolar method allows an increase in the number of electrodes on the abdominal wall, which increases occasions allowing measurements of fetus electrocardiogram signals. Such a technique is disclosed in, for example, the Patent Document 4 and the Non-Patent Document 5 each described below. However, as compared with bipolar electrode method, in the unipolar method, much noise is mixed into the circuit, and hence, in order to avoid it, shield lines are mainly used for use in reducing the noise. However, because the measurement must be performed in a shield room in many cases, this method is far from a device actually useful in monitoring on a clinical level. The unipolar method further involves the following problems: the electrode must be moved to find a position where and the electrode resistance is low and fetus electrocardiogram is satisfactorily measured; even if a measurement is being performed at an electrode position that has been found after the measurer's all efforts, there occurs the need to reattach the electrode when the fetus moves in fetus activities; and in cases wherein a mother as a subject is nervous, measurement is frequently impossible.
Therefore, monitoring for continuous long time including a time period of fetus movements in an ambulant or hospitalized pregnant woman is difficult. Moreover, even if the noise problem is factored out, in order to entirely cover the abdominal wall of the mother body, 100 to 200 electrodes are theoretically needed, which indicates that it is virtually impracticable to cover monitoring during fetus movements. However, as in the case of an exercise stress test performed in an adult electrocardiogram measurement, in a fetus also, an abnormality such as myocardial ischemia would be observed more frequently during fetus movements than at rest. Therefore, an electrode method usable during fetus movements as well as at its rest is desired. Moreover, since cases requiring urgent diagnoses often happen when the electrode method is used in clinical practice, a device usable for ambulant or hospitalized pregnant women, and adaptable to cases as many as possible, is desired.
However, there has hitherto been no implementation of low-noise and high-sensitivity fetus electrocardiogram signal measuring method and its device, as described above, capable of continuously measuring fetus electrocardiogram signals even during fetus movements, and usable for an ambulant or hospitalized pregnant woman without the need for reattaching the electrodes.    [Patent Document 1] PCT Japanese Translation Patent Publication No. 2002-532182    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2004-121733    [Patent Document 3] PCT Japanese Translation Patent Publication No. 2002-538872    [Patent Document 4] PCT Japanese Translation Patent Publication No. 2005-503883    [Non-patent document 1] Rosen KG: Fetus ECG waveform analysis in labour. Fetus monitoring. Physiology and techniques of antenatal and intrapartum assessment. Ad. Spencer JAD Castle House Publications. pp 184-187, 1989.
[Non-patent document 2] Ogawa Teruyuki: Time-series autoregressive analysis of normal fetus/neonate electrocardiogram R-R intervals. Actuality of time series II written and edited by Akaike Koji and Kitagawa Genshiro, 4th Chap. pp. 61-74, Asakura Shoten, 1995.    [Non-patent document 3] Peters M, et al: Monitoring the fetus heart non-invasively: a review of methods. J. Perinat. Med. 29(2001), pp. 408-416.    [Non-patent document 4] Zarzoso V and Nandi A K: Noninvasive fetus electrocardiogram extraction: Blind separation versus adaptive noise cancellation. IEEE Trans. Biomed. Eng. 48, 12-18, 2001.    [Non-patent document 5] Taylor M J O, et al.: Non-invasive fetus electrocardiography in singleton and multiple pregnancies. BJOG, 110, 668-78, 2003.    [Non-patent document 6] Yazaki Hiroyuki, Higuchi Masataka, Kyoso Masaki, Ishijima Masayuki: Unconscious sleep monitoring using large-area capacitor electrodes. The 46th Annual Meeting of Japan Soc, ME&BE, April, 2007, PS1-11-7.    [Non-patent document 7] Ishida Shuhei, Shiozawa Naruhiro, Fujiwara Yoshihisa, Makikawa Masaaki:    Electrocardiogram measurement during sleep with clothes worn using capacitively-coupled electrodes arranged at bedside. The 46th Annual Meeting of Japan Soc, ME&BE, April, 2007, PS1-11-8.